How to make many to many relation in browser sync storage?

Question

I'm creating an extension which is gonna allow users to tag comments. In an SQL database I would create tables like that:

comments:
  id int
  text string

tags:
  id int
  tag_name string

comment_tags:
  comment_id int
  tag_id int

But WebExtensions API provides us pretty few methods(https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage/sync#Methods) with "key value" access

I have some ideas like to store this as a bunch of keys, like so:

{
  "comment_1": { comment data },
  "comment_2": { comment data },
  "tag_1": { tag data },
  "tag_2": { tag data },
}

where 1 and 2 are ids and to store relations either in comment's and tag's data or as another key "comment_tags_1_2". But both ways are inconvinient.

What is a typical way of solving this task?